To increase the number of channels wavelength-division multiplexed tele communications systems can carry, researchers at Lucent Technologies Bell Labs (Holmdel, NJ) have developed a wavelength-division multiplexer (WDM) transmitter capable of generating at least 206 channels using a single laser, a single chirped fiber, and a single modulator. The system was described in two papers at OFC `97 in Dallas, TX, in February. "This is a new world record," claims Wayne Knox, of Bell Labs. Sixteen-wavelength systems have just become commercially available.
The high channel volume was achieved using a modelocked erbium-doped fiber laser and a chirped-pulse WDM technique (CPWDM) developed by Knox, Martin Nuss, Luc Boivin, Jason Stark, and Steve Cundiff. A key feature of the technique is its capability to generate and encode data on a large number of channels.
The technology is easy to understand, says Knox. Imagine a piano, with each of its 88 keys corresponding to a different wavelength. "Present WDM systems are built like 88 separate pianos, each with one key that plays only one note," explains Knox. "We pulled all the keys together into one system. The tricky part was how to keep separate all the wavelengths being emitted at the same time. The process we developed is like taking your finger and running it along the piano keys from the lowest to the highest—we start at the lowest note, play it for a short time, go to the next note, play it for a short time, then the next."
In the case of optical transmission, each wavelength propagates through a fiber at its own speed. Shorter ("bluer") wavelengths are emitted first, redder ones last. The wavelengths are encoded onto each channel sequentially in time, starting on the blue side and sliding to red.
The 206-channel wavelength-division-multiplexed transmitter is based on a broad-bandwidth (in excess of 70 nm) femtosecond laser emitting around 1.55 µm at a repetition rate of 36.7 MHz, together with a time-division-multiplexed electro-absorption modulator. The channel spacing is about 37 GHz (0.3 nm), and the bit rate in each channel is 36.7 Mbit/s, with wavelengths ranging from 1535.3 to 1596.3 nm.
The spectrum of each pulse is mapped onto the time axis by propagation through a single-mode fiber with a total dispersion of 340 ps/nm. This stretches out the pulses to a duration of about 24.2 ns and results in a linear relationship between wavelength and time delay within each pulse. A TDM electro-absorption modulator with a 12-GHz bandwidth is inserted at the output of the chirping fiber to define and encode the data onto each channel sequentially in time.1
The chirp converts the TDM pattern into a WDM modulation, with the states "1" and "0" corresponding to frequency bands of high and low intensity, respectively. Out of the 271 possible wavelength slots defined by the modulator, 206 can be identified with less than 3-dB channel-to-channel power variation. By encoding a different 271-bit TDM word onto each chirped pulse, independent messages can be sent through each channel at a bit rate equal to the laser repetition rate.
"There is almost no limit to the number of channels that can be generated," says Knox. But before the technology can be deployed, a fully telecommunications-capable pulsed broadband light source must be developed. Knox and his colleagues believe that higher bit rates can be obtained by using a laser with a higher repetition rate. The optical components required to build a 206-channel WDM network are not all commercially available.
REFERENCES
1. Luc Boivin et al., OFC `97 Technical Digest, 276, paper ThI2.